Hemangioblast, hemogenic endothelium, and primitive versus definitive hematopoiesis
2016; Elsevier BV; Volume: 49; Linguagem: Inglês
10.1016/j.exphem.2016.12.009
ISSN1873-2399
AutoresGeorges Lacaud, Valérie Kouskoff,
Tópico(s)Pluripotent Stem Cells Research
Resumo•Primitive hematopoiesis encompasses the earliest wave of blood emergence, occurring at approximately E7.25 in the yolk sac.•There is still no conclusive in vivo experimental evidence demonstrating the presence, or lack thereof, of hemangioblasts.•All blood cells emerge from hemogenic endothelial-expressing cells through an endothelial-to-hematopoietic transition.•Whether these endothelial-expressing cells are termed “angioblast” or “endothelium” depends on their localization within the developing vasculature. The types of progenitors generated during the successive stages of embryonic blood development are now fairly well characterized. The terminology used to describe these waves, however, can still be confusing. What is truly primitive? What is uniquely definitive? These questions become even more challenging to answer when blood progenitors are derived in vitro upon the differentiation of embryonic stem cells or induced pluripotent stem cells. Similarly, the cellular origin of these blood progenitors can be controversial. Are all blood cells, including the primitive wave, derived from hemogenic endothelium? Is the hemangioblast an in vitro artifact or is this mesoderm entity also present in the developing embryo? Here, we discuss the latest findings and propose some consensus relating to these controversial issues. The types of progenitors generated during the successive stages of embryonic blood development are now fairly well characterized. The terminology used to describe these waves, however, can still be confusing. What is truly primitive? What is uniquely definitive? These questions become even more challenging to answer when blood progenitors are derived in vitro upon the differentiation of embryonic stem cells or induced pluripotent stem cells. Similarly, the cellular origin of these blood progenitors can be controversial. Are all blood cells, including the primitive wave, derived from hemogenic endothelium? Is the hemangioblast an in vitro artifact or is this mesoderm entity also present in the developing embryo? Here, we discuss the latest findings and propose some consensus relating to these controversial issues. During embryogenesis, the hematopoietic system is established in successive waves that are temporally and spatially restricted, with each giving rise to specific blood progenitors (Fig. 1) [1Costa G. Kouskoff V. Lacaud G. Origin of blood cells and HSC production in the embryo.Trends Immunol. 2012; 35: 215-223Abstract Full Text Full Text PDF Scopus (64) Google Scholar]. Around embryonic day 7.25 (E7.25), shortly after gastrulation, the onset of blood emergence takes place in the yolk sac, where extraembryonic mesoderm differentiates to form blood islands [2Ferkowicz M.J. Yoder M.C. Blood island formation: longstanding observations and modern interpretations.Exp Hematol. 2005; 33: 1041-1047Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar]. The first blood precursors generated at this stage of development give rise to primitive erythroid precursors, erythroid cells with unique characteristics that are only found during early embryogenesis and the role of which is to deliver oxygen rapidly to the quickly expanding embryo [3Palis J. Robertson S. Kennedy M. Wall C. Keller G. Development of erythroid and myeloid progenitors in the yolk sac and embryo proper of the mouse.Development. 1999; 126: 5073-5084Crossref PubMed Google Scholar, 4Palis J. Ontogeny of erythropoiesis.Curr Opin Hematol. 2008; 15: 155-161Crossref PubMed Scopus (115) Google Scholar]. Macrophage and megakaryocyte progenitors are also generated during this earliest wave of blood emergence [3Palis J. Robertson S. Kennedy M. Wall C. Keller G. Development of erythroid and myeloid progenitors in the yolk sac and embryo proper of the mouse.Development. 1999; 126: 5073-5084Crossref PubMed Google Scholar, 5Tober J. Koniski A. McGrath K.E. et al.The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis.Blood. 2007; 109: 1433-1441Crossref PubMed Scopus (209) Google Scholar]. The next wave of blood specification starts a day later, at approximately E8.25, with the emergence of erythromyeloid progenitors (EMPs) within the yolk sac [6McGrath K.E. Frame J.M. Fegan K.H. et al.Distinct sources of hematopoietic progenitors emerge before HSCs and provide functional blood cells in the mammalian embryo.Cell Rep. 2015; 11: 1892-1904Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar]. These EMPs produce definitive erythrocytes and most myeloid lineages. By E9.0 to E9.5, B and T lymphoid progenitors are generated in the yolk sac and in the intraembryonic para-aortic splanchnopleura region [7Yoshimoto M. Montecino-Rodriguez E. Ferkowicz M.J. et al.Embryonic day 9 yolk sac and intra-embryonic hemogenic endothelium independently generate a B-1 and marginal zone progenitor lacking B-2 potential.Proc Natl Acad Sci U S A. 2011; 108: 1468-1473Crossref PubMed Scopus (187) Google Scholar, 8Yoshimoto M. Porayette P. Glosson N.L. et al.Autonomous murine T-cell progenitor production in the extra-embryonic yolk sac before HSC emergence.Blood. 2012; 119: 5706-5714Crossref PubMed Scopus (107) Google Scholar]. All types of fetal and adult T cells are produced by these early progenitors, including the αβ and γδ subsets [8Yoshimoto M. Porayette P. Glosson N.L. et al.Autonomous murine T-cell progenitor production in the extra-embryonic yolk sac before HSC emergence.Blood. 2012; 119: 5706-5714Crossref PubMed Scopus (107) Google Scholar]. In the case of B lymphocytes, the potential of this first wave of progenitors is restricted to the production of innate-type B1 and marginal zone B-cell subsets [7Yoshimoto M. Montecino-Rodriguez E. Ferkowicz M.J. et al.Embryonic day 9 yolk sac and intra-embryonic hemogenic endothelium independently generate a B-1 and marginal zone progenitor lacking B-2 potential.Proc Natl Acad Sci U S A. 2011; 108: 1468-1473Crossref PubMed Scopus (187) Google Scholar, 9Montecino-Rodriguez E. Dorshkind K. B-1 B cell development in the fetus and adult.Immunity. 2012; 36: 13-21Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar]. Recent studies have also established that tissue-resident macrophages of the brain, lung, and liver are generated during these earliest waves of hematopoietic development [10Ginhoux F. Guilliams M. Tissue-resident macrophage ontogeny and homeostasis.Immunity. 2016; 44: 439-449Abstract Full Text Full Text PDF PubMed Scopus (819) Google Scholar], macrophage populations that persist in the adult organism throughout life. It is only by E10.5 that the first long-term multilineage adult-engrafting hematopoietic stem cells (HSCs) are generated [11Medvinsky A. Dzierzak E. Definitive hematopoiesis is autonomously initiated by the AGM region.Cell. 1996; 86: 897-906Abstract Full Text Full Text PDF PubMed Scopus (1124) Google Scholar]. HSCs emerge within the major arteries of the developing embryo, including the dorsal aorta, the vitelline, and umbilical arteries [12de Bruijn M.F. Speck N.A. Peeters M.C. Dzierzak E. Definitive hematopoietic stem cells first develop within the major arterial regions of the mouse embryo.EMBO J. 2000; 19: 2465-2474Crossref PubMed Scopus (439) Google Scholar]. By E11.5, HSCs are also found in the yolk sac and placenta [13Gekas C. Dieterlen-Lievre F. Orkin S.H. Mikkola H.K. The placenta is a niche for hematopoietic stem cells.Dev Cell. 2005; 8: 365-375Abstract Full Text Full Text PDF PubMed Scopus (480) Google Scholar, 14Ottersbach K. Dzierzak E. The murine placenta contains hematopoietic stem cells within the vascular labyrinth region.Dev Cell. 2005; 8: 377-387Abstract Full Text Full Text PDF PubMed Scopus (344) Google Scholar]; however, whether this occurs through de novo generation or through the circulation of HSCs generated elsewhere is still unknown. Newly generated HSCs migrate to the fetal liver, where they undergo massive expansion; by E16.5, they start to colonize the bone marrow, where they will reside throughout adult life, self-renewing and producing a continuous supply of all blood lineages. In the developing mouse embryo, primitive hematopoiesis is most often defined as the initial wave of blood cell production taking place at approximately E7.25 in the blood islands within the yolk sac [2Ferkowicz M.J. Yoder M.C. Blood island formation: longstanding observations and modern interpretations.Exp Hematol. 2005; 33: 1041-1047Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, 5Tober J. Koniski A. McGrath K.E. et al.The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis.Blood. 2007; 109: 1433-1441Crossref PubMed Scopus (209) Google Scholar, 15Lux C.T. Yoshimoto M. McGrath K. Conway S.J. Palis J. Yoder M.C. All primitive and definitive hematopoietic progenitor cells emerging before E10 in the mouse embryo are products of the yolk sac.Blood. 2008; 111: 3435-3438Crossref PubMed Scopus (211) Google Scholar]. As stated above, this first wave gives rise to primitive erythrocytes, macrophages, and megakaryocytes (Fig. 2). All subsequent waves of blood emergence in the embryo, from E8.25 on, are defined as definitive hematopoiesis. This includes EMPs produced in the yolk sac, which give rise to definitive erythrocytes, macrophages, megakaryocytes, and other myeloid lineages; early T and B progenitors produced in the yolk sac and para-aortic splanchnopleura; and HSCs produced in the dorsal aorta, vitelline, and umbilical arteries (Fig. 2). Alternate definitions for primitive hematopoiesis can be found in the literature: for example, primitive hematopoiesis, representing all precirculation hematopoiesis [16Gritz E. Hirschi K.K. Specification and function of hemogenic endothelium during embryogenesis.Cell Mol Life Sci. 2016; 73: 1547-1567Crossref PubMed Scopus (72) Google Scholar]; primitive hematopoiesis, encompassing all blood cells produced before HSC emergence; or primitive hematopoiesis, representing all blood lineages except definitive erythrocytes, T cells, and HSCs [17Sturgeon C.M. Ditadi A. Awong G. Kennedy M. Keller G. Wnt signaling controls the specification of definitive and primitive hematopoiesis from human pluripotent stem cells.Nat Biotechnol. 2014; 32: 554-561Crossref PubMed Scopus (248) Google Scholar]. However, these alternate definitions of primitive hematopoiesis are not used widely and are not based on strong scientific evidence. Primitive erythrocytes are easily distinguishable from definitive erythrocytes by their cellular and molecular characteristics. They are much larger than definitive erythrocytes and predominantly express embryonic forms of globin [18Palis J. Primitive and definitive erythropoiesis in mammals.Front Physiol. 2014; 5: 3Crossref PubMed Scopus (237) Google Scholar, 19Kingsley P.D. Malik J. Emerson R.L. et al.“Maturational” globin switching in primary primitive erythroid cells.Blood. 2006; 107: 1665-1672Crossref PubMed Scopus (118) Google Scholar]. However, like definitive erythroid cells, primitive erythroid cells ultimately enucleate at late stage of maturation [20Kingsley P.D. Malik J. Fantauzzo K.A. Palis J. Yolk sac-derived primitive erythroblasts enucleate during mammalian embryogenesis.Blood. 2004; 104: 19-25Crossref PubMed Scopus (181) Google Scholar]. In contrast, macrophages and megakaryocytes generated during the primitive wave of hematopoiesis are barely distinguishable from their definitive counterparts [5Tober J. Koniski A. McGrath K.E. et al.The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis.Blood. 2007; 109: 1433-1441Crossref PubMed Scopus (209) Google Scholar]. Defining and characterizing primitive hematopoiesis in the embryo is straightforward because this wave is restricted in time and space. However, defining this primitive wave during the in vitro differentiation of embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) is more challenging and only primitive erythroid precursors can be identified with certainty as being part of this primitive wave. Macrophages and megakaryocytes can be equally generated from primitive or definitive hematopoiesis. One might ask why it is so important to have a clear understanding and definition of primitive versus definitive or successive waves of blood emergence during the in vitro differentiation process. A long-standing quest in the field of ESC differentiation to blood has been the in vitro generation of HSCs [21Garcia-Alegria E. Menegatti S. Batta K. Cuvertino S. Florkowska M. Kouskoff V. Emerging concepts for the in vitro derivation of murine haematopoietic stem and progenitor cells.FEBS Lett. 2016; 590: 4116-4125Crossref PubMed Scopus (6) Google Scholar, 22Batta K. Menegatti S. Garcia-Alegria E. Florkowska M. 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Definitive hematopoiesis emergence has been monitored by following the generation of T-lymphocyte progenitors [17Sturgeon C.M. Ditadi A. Awong G. Kennedy M. Keller G. Wnt signaling controls the specification of definitive and primitive hematopoiesis from human pluripotent stem cells.Nat Biotechnol. 2014; 32: 554-561Crossref PubMed Scopus (248) Google Scholar, 26Kennedy M. Awong G. Sturgeon C.M. et al.T lymphocyte potential marks the emergence of definitive hematopoietic progenitors in human pluripotent stem cell differentiation cultures.Cell Rep. 2012; 2: 1722-1735Abstract Full Text Full Text PDF PubMed Scopus (268) Google Scholar]. Although T cells clearly represent a definitive lineage, they are not indicative of HSC emergence because T-cell progenitors are generated at E9.5 in the yolk sac and embryo proper one full day before the emergence of the first HSCs [8Yoshimoto M. Porayette P. Glosson N.L. et al.Autonomous murine T-cell progenitor production in the extra-embryonic yolk sac before HSC emergence.Blood. 2012; 119: 5706-5714Crossref PubMed Scopus (107) Google Scholar]. It is therefore clear that T-cell potential cannot be used to track HSC emergence. In fact, only one single hematopoietic lineage appears to be specifically generated from adult-repopulating HSCs and it is the B2-adaptive B-cell lineage. The emergence of B-cell progenitors before HSC generation is restricted to the formation of innate-specific B1 and marginal zone B cells [7Yoshimoto M. Montecino-Rodriguez E. Ferkowicz M.J. et al.Embryonic day 9 yolk sac and intra-embryonic hemogenic endothelium independently generate a B-1 and marginal zone progenitor lacking B-2 potential.Proc Natl Acad Sci U S A. 2011; 108: 1468-1473Crossref PubMed Scopus (187) Google Scholar, 9Montecino-Rodriguez E. Dorshkind K. B-1 B cell development in the fetus and adult.Immunity. 2012; 36: 13-21Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar]. Therefore, one might speculate that, in order to monitor HSC emergence in vitro, it may be best to monitor the emergence of B2 lymphoid cells. Interestingly, there are very few, if any, reports of HSC generation in vitro and there are equally few reports of B-cell generation in vitro [27Vodyanik M.A. Bork J.A. Thomson J.A. Slukvin II, Human embryonic stem cell-derived CD34+ cells: efficient production in the coculture with OP9 stromal cells and analysis of lymphohematopoietic potential.Blood. 2005; 105: 617-626Crossref PubMed Scopus (505) Google Scholar, 28Carpenter L. Malladi R. Yang C.T. et al.Human induced pluripotent stem cells are capable of B-cell lymphopoiesis.Blood. 2011; 117: 4008-4011Crossref PubMed Scopus (59) Google Scholar, 29French A. Yang C.T. Taylor S. Watt S.M. Carpenter L. Human induced pluripotent stem cell-derived B lymphocytes express sIgM and can be generated via a hemogenic endothelium intermediate.Stem Cells Dev. 2015; 24: 1082-1095Crossref PubMed Scopus (33) Google Scholar]. Furthermore, whether B1 or B2 lymphoid subsets were obtained in these studies was not determined. In our recent study using mouse ESCs [30Pearson S. Cuvertino S. Fleury M. Lacaud G. Kouskoff V. In vivo repopulating activity emerges at the onset of hematopoietic specification during embryonic stem cell differentiation.Stem Cell Reports. 2015; 4: 431-444Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar], we did report the in vitro generation of multilineage engrafting blood progenitors. Strikingly, B2 lymphocytes were generated from these engrafting progenitors, suggesting that they might be true HSCs but with limited self-renewal capacity. The term “hemangioblast” was initially coined by Murray in 1932 [31Murray P.D.F. The development in vitro of the blood of the early chick embryo.Proc Roy Soc London. 1932; 11: 497-521Crossref Google Scholar] and referred to a mass of cells derived from the primitive streak mesoderm that contain both endothelium and blood cells. This was meant to complement and contrast the term “angioblast,” which was discussed previously by Sabin [32Sabin F. Studies on the origin of blood vessels and of red corpuscles as seen in the living blastoderm of the chick during the second day of incubation.Contributions to Embryology. 1920; 9: 213-262Google Scholar] and only referred to vessels or endothelium. The hemangioblast, as originally described by Murray, was not a clonal mesoderm precursor giving rise to both blood and endothelium. The concept of the hemangioblast as a clonal precursor gained traction in the late 1990's, when it was shown that single mesodermal cells isolated from in vitro differentiating mouse ESCs could give rise to both blood cells and endothelium [33Choi K. 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This has led to the proposition that hemangioblast is a state of competency that is never fulfilled in vivo due to the restriction and constraint imposed by the microenvironment [39Amaya E. The hemangioblast: a state of competence.Blood. 2013; 122: 3853-3854Crossref PubMed Scopus (7) Google Scholar]. In vivo lineage tracing in the mouse embryo has so far failed to demonstrate the existence of hemangioblasts. Ueno et al. [40Ueno H. Weissman I.L. Clonal analysis of mouse development reveals a polyclonal origin for yolk sac blood islands.Dev Cell. 2006; 11: 519-533Abstract Full Text Full Text PDF PubMed Scopus (187) Google Scholar], using multicolor chimera embryos and FLK1-cre lineage tracing, concluded that blood islands were derived from multiple precursors and that most blood cells did not derive from FLK1-expressing precursors. However, given that hemangioblasts are mostly localized in the posterior primitive streak [35Huber T.L. Kouskoff V. Fehling H.J. Palis J. Keller G. Haemangioblast commitment is initiated in the primitive streak of the mouse embryo.Nature. 2004; 432: 625-630Crossref PubMed Scopus (527) Google Scholar], by the time these mesoderm progenitors reach the yolk sac to form blood islands, they have already divided and their daughter cells have already initiated their fate specification toward the endothelium or blood. Given the highly migratory behaviours of cells in the gastrulating embryo [41Tanaka Y. Sanchez V. Takata N. et al.Circulation-independent differentiation pathway from extraembryonic mesoderm toward hematopoietic stem cells via hemogenic angioblasts.Cell Rep. 2014; 8: 31-39Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 42Eliades A. Wareing S. 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This specialized endothelial population, termed the “hemogenic endothelium,” is thought to give rise to blood cells through an endothelial-to-hematopoietic transition rather than through an asymmetric division [51Eilken H.M. Nishikawa S. Schroeder T. Continuous single-cell imaging of blood generation from haemogenic endothelium.Nature. 2009; 457: 896-900Crossref PubMed Scopus (452) Google Scholar]. By definition, a hemogenic endothelium is an endothelial cell that has the potential to become a blood cell, is characterized by an endothelial-specific gene expression signature and endothelial-specific cell morphology, and is localized within the endothelial layer of a blood vessel. Hemogenic endothelium as a cell population giving rise to blood cells has now been described in most species studied to date. In the mouse embryo, E8.25 yolk sac EMPs [56Frame J.M. Fegan K.H. Conway S.J. McGrath K.E. Palis J. Definitive hematopoiesis in the yolk sac emerges from Wnt-responsive hemogenic endothelium independently of circulation and arterial identity.Stem Cells. 2016; 34: 431-444Crossref PubMed Scopus (97) Google Scholar], E9.5 T and B progenitors [7Yoshimoto M. Montecino-Rodriguez E. Ferkowicz M.J. et al.Embryonic day 9 yolk sac and intra-embryonic hemogenic endothelium independently generate a B-1 and marginal zone progenitor lacking B-2 potential.Proc Natl Acad Sci U S A. 2011; 108: 1468-1473Crossref PubMed Scopus (187) Google Scholar, 8Yoshimoto M. Porayette P. Glosson N.L. et al.Autonomous murine T-cell progenitor production in the extra-embryonic yolk sac before HSC emergence.Blood. 2012; 119: 5706-5714Crossref PubMed Scopus (107) Google Scholar], and E10.5 intraembryonic progenitors and HSCs [53Boisset J.C. van Cappellen W. Andrieu-Soler C. Galjart N. Dzierzak E. Robin C. In vivo imaging of haematopoietic cells emerging from the mouse aortic endothelium.Nature. 2010; 464: 116-120Crossref PubMed Scopus (617) Google Scholar] have all been shown to emerge from hemogenic endothelium (Fig. 2). In contrast, the cellular origin of the E7.25 wave of primitive hematopoiesis is still disputed. It seems unclear whether primitive hematopoiesis emerges directly from mesoderm, from hemogenic endothelium, or from another type of precursor. This first wave of blood development occurs before the formation of the vasculature and therefore cannot emerge from a bona fide hemogenic endothelium. However, there is clear evidence in the literature demonstrating that primitive hematopoiesis does arise from precursors expressing endothelial markers including TIE2, VE-cadherin, and CD31 [45Lancrin C. Sroczynska P. Stephenson C. Allen T. Kouskoff V. Lacaud G. 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Understanding how the hematopoietic system develops during embryogenesis will provide critical knowledge to translate to the in vitro derivation of blood progenitors for use in the clinical setting. However, although there are many similarities between in vitro and in vivo blood cell emergence, there are also clear differences. An intriguing observation in our recent study on in vitro blood emergence was that all progenitors, including primitive and definitive erythrocytes, myeloid, T cells, B cells, and engrafting cells, were generated at the same time from the mesoderm, suggesting that there are no sequential waves of blood specification in vitro [30Pearson S. Cuvertino S. Fleury M. Lacaud G. Kouskoff V. In vivo repopulating activity emerges at the onset of hematopoietic specification during embryonic stem cell differentiation.Stem Cell Reports. 2015; 4: 431-444Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar]. 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Recent studies have suggested that endothelial progenitors generated around E7.0 to E7.5 in the extraembryonic yolk sac migrate to intraembryonic sites and contribute to the formation of the dorsal aorta [41Tanaka Y. Sanchez V. Takata N. et al.Circulation-independent differentiation pathway from extraembryonic mesoderm toward hematopoietic stem cells via hemogenic angioblasts.Cell Rep. 2014; 8: 31-39Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 42Eliades A. Wareing S. Marinopoulou E. et al.The hemogenic competence of endothelial progenitors is restricted by runx1 silencing during embryonic development.Cell Rep. 2016; 15: 2185-2199Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar]. Furthermore, we showed that, in these migrating endothelium progenitors, blood specification was impaired through the active silencing of Runx1 via a BMI1-dependent mechanism [42Eliades A. Wareing S. 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